Optical communication systems are known, e.g., from B. Wedding et al, "10 Gbit/s to 260000 Subscribers Using Optical Amplifier Distribution Network", Contribution for ICC/Supercomm '92, Optical Communications 300 Level Session, "Impact of Optical Amplifiers on Network Architectures". A fiber-optic amplifier is shown in detail in EP 0 457 349 A2, for example.
In an optical communication system, fiber-optic amplifiers generally serve to amplify an optical signal being guided in a fiber-optic link.
Fiber-optic amplifiers are connected to an optical waveguide by splices or fiber-optic connectors. Splices or fiber-optic connectors are also used to connect one optical waveguide to another.
An amplifying section of an optical waveguide contained in a fiber-optic amplifier is doped with ions of a rare-earth element, such as Er.sup.3+. Pump light emitted by a pump source is coupled via a coupler into the amplifying section of optical waveguide. Through the pump light, the erbium ions are raised from a ground state to an excited state, from which they drop back, through either spontaneous or stimulated emission, to the ground state. The stimulated emission is excited by the optical signal passing through the amplifying section of optical waveguide. The spontaneous emission is also amplified in the amplifying section of optical waveguide; this amplified spontaneous emission (ASE) propagates in and opposite to the direction of transmission of the optical signal and is the cause of the noise internal to a fiber-optic amplifier.
As regards the use of optical isolators, there are differences in the literature. In the patent specification referred to above, two optical isolators are used, one at the input and one at the output. In the above-mentioned paper by Wedding, B. et al, an optical isolator is employed only at the output. Optical isolators ensure stable amplifier operation but degrade the amplifier's noise characteristics due to their insertion loss. Elimination of the optical isolator at the signal input would therefore have a favourable effect on the costs and noise characteristics of the fiber-optic amplifier.
It is therefore desirable to omit the optical isolator at the input. It has turned out, however, that system components, e.g., photodiodes, may be damaged when a fiber-optic connector is being separated.